Setup Devise in your application.rb instead of the models

application.rb

module MyApp
  class Application < Rails::Application
    config.to_prepare do
      User.devise :database_authenticatable, :recoverable, :rememberable,
                  :trackable, :validatable

      Admin.devise :database_authenticatable, :recoverable, :rememberable,
                   :trackable, :validatable
    end
  end
end